As sensors for a feedback use in an automatic control system, for example, sensors for detecting the absolute values (analog amounts) of a rotational angle and a displacement amount are known, and a typical one of these sensors is a potentiometer. The potentiometer is classified into a contact type shown in FIGS. 1 and 2, and a non-contact type shown in FIGS. 3 and 4.
As shown in FIGS. 1 and 2, a contact type potentiometer comprises a circular base 2 fixed with a cylindrical wall 2a to the inner surface of which a resistor member 1 is adhered. A rotational shaft 4 is supported by the circular base 2. A brush 7 is attached to the rotational shaft 4 via a movable arm 6, and a slip ring 8 is fixed to the rotational shaft 4. A V-shaped conductive wire 9 is connected to an output terminal 10, and slidably contacts with a recess groove 8a formed by notching the slip ring 8.
The resistor member 1 consists of a metal resistor or conductive plastic to have a C-shaped cross-section, and its two terminals are respectively connected to input terminals 3.3. The rotational shaft 4 is pivotally supported on the bottom portion of the base 2 via a bearing 5, and is located at the central portion of the resistor member 1. The brush 7 is attached to the distal end of the movable arm 6, and slidably contacts with the inner circumferential surface of the resistor member 1. The movable arm 6 and the slip ring 8 located above the arm 6 are electrically connected to each other via the rotational shaft 4.
The above-mentioned contact type potentiometer is used by applying a DC voltage across the input terminals 3.3. When the brush 7 rotates and is displaced together with the rotational shaft 4, the voltage from the output terminal 10 changes. More specifically, the absolute value of the voltage dividing ratio of the DC voltage by the resistor member 1 is obtained in accordance with the position of the brush 7, and hence, the absolute value of the rotational angle or the displacement amount of the rotational shaft 4 can be detected.
This potentiometer is not influenced by a change in temperature, and can be used in an environment in a wide temperature range from -40.degree. C. to +150.degree. C. since it detects the position of the brush 7 using the voltage dividing ratio of the DC voltage in accordance with the resistance dividing ratio of the resistor member 1. However, detection precision is impaired due to wear of the resistor member 1 by the brush 7, and the service life of the potentiometer is shortened. In addition, a torque upon sliding is undesirably large.
In order to prolong the service life, a potentiometer including a non-contact detection portion is proposed. As shown in FIGS. 3 and 4, this potentiometer comprises a disk-shaped base 2 having a cylindrical wall 2a, and two magnetoresistive elements 11.11 are arranged on the base 2.
The two magnetic resistance elements 11.11 are designed to have a substantially semi-arcuated shape, and are arranged on the bottom portion of the base 2 to have a common center of curvature. These two magnetic resistance elements 11.11 are commonly connected to an output terminal 10 via a conductive wire. Input terminals 3.3 are respectively connected to the end portions of these elements 11.11 via conductive wires.
A rotational shaft 4 is pivotally inserted in the bottom portion of the base 2 via a bearing 5, and is located at the center of curvature of the two magnetic resistance elements 11.11. Furthermore, a permanent magnet 13 having a substantially semi-circular shape is attached as a magnetic field generating source to the circumferential surface of the rotational shaft 4, and opposes to the two magnetic resistance elements 11.11 via gaps. The permanent magnet 13 rotates in an facing state to the two magnetic resistance elements 11.11 via gaps upon rotation of the rotational shaft 4, thereby changing the amount of a magnetic field applied to the two magnetic resistance elements 11.11.
The above-mentioned potentiometer is used by applying a DC voltage across the input terminals 3.3. The potentiometer also utilizes a change in internal resistance of the magnetic resistance elements 11.11 upon application of a magnetic field to the magnetic resistance elements 11.11. More specifically, when the permanent magnet 13 rotates, the amount of a magnetic field applied to the magnetic resistance elements 11.11 facing to the permanent magnet 13 changes, and the resistances of the elements 11.11 increase/decrease accordingly. The potential of the output terminal 10 changes due to such increase/decrease in resistance, and hence, the position of the permanent magnet 13 is determined.
This non-contact type potentiometer has a long service life since the detection portion does not suffer from mechanical deterioration. However, the magnetic characteristics of the permanent magnet 13 easily deteriorate due to a change over time, resulting in a problem of detection precision. Furthermore, since the magnetic resistance elements 11.11 comprising semiconductors have poor temperature characteristics, temperature compensation must be taken into consideration when the potentiometer is used in a wide temperature range.
In order to realize accurate automatic control even in a harsh environment, it is important to widen the applicable temperature range of a potentiometer as a sensor for detecting the absolute value of a rotational angle or a displacement amount, and to prolong the service life of the potentiometer. However, in the potentiometer including the mechanical contact type detection portion, the temperature range can be widened, but it is difficult to prolong the service life. In contrast to this, in the potentiometer including the non-contact type detection portion, the service life can be prolonged, but it is difficult to widen the temperature range. Thus, both of these potentiometers have merits and demerits.